This invention relates generally to feedthrough capacitor terminal pin assemblies and related methods of construction, particularly of the type used in active implantable medical devices (AIMD), such as cardiac pacemakers, implantable hearing devices, implantable cardioverter defibrillators, neurostimulators, drug pumps and the like. Electromagnetic interference (EMI) feedthrough filter capacitors are typically used in such applications to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More specifically, this invention relates to processes and apparatuses for installing feedthrough capacitors to terminal pin assemblies utilizing conductive, resiliently flexible contact springs. This invention is particularly designed for use in cardiac pacemakers and cardioverter defibrillators. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules, wherever it is desirable to preclude entry of EMI into a shielded housing. The simplified electrical contact method as described herein is applicable both to hermetically sealed housings and non-hermetically sealed housings and bulkheads.
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in active implantable medical devices, such as cardiac pacemakers, defibrillators or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. See, for example, U.S. Pat. No. 5,333,095, the contents of which are incorporated herein. The feedthrough terminal pins are typically connected to one or more lead wires which can undesirably act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device. In the prior art devices, the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device.
In prior art devices, a feedthrough capacitor is attached to the ferrule or insulator of the terminal of an active implantable medical device using various attachment methods. For example, thermal-setting conductive adhesives, such as conductive polyimides, solders, welds, brazes and the like, are all used to mechanically and electrically make connections to the feedthrough capacitor. With reference to U.S. Pat. No. 5,333,095, a feedthrough capacitor is surface mounted onto the hermetic terminal subassembly. It is desirable to have a high temperature electrical connection between the lead wires and the inside diameter holes of a feedthrough capacitor. It is also desirable to have a high temperature electrical connection around the outside diameter or perimeter of the capacitor to the ferrule. In most of the prior art applications, including that shown in U.S. Pat. No. 5,333,095, the electrical connection material is a thermal-setting conductive polyimide such as that manufactured by Ablestick. Conductive polyimide is typically inserted using a microsyringe into the annular space between the lead wires and the inside diameter feedthrough holes of the feedthrough capacitor. Multiple centrifuging steps are normally required to pack and densify the thermal-setting conductive polyimide. It is important that the thermal-setting conductive polyimide not have large voids or cavities.
Because of the need to inject and then centrifuge the conductive polyimide, it is important that this material not be allowed to flow out underneath the capacitor where it could cause short circuits. Accordingly, in prior art devices there is an insulating washer (typically of a non-conductive polyimide material) that is disposed between the ceramic capacitor and a mounting surface of a terminal pin-supporting alumina insulator. In manufacturing the terminal pin feedthrough subassembly, the capacitor is seated against this non-conductive polyimide washer and then cured.
However, complications follow from the use of the conductive polyimide; that is, after the conductive polyimide is centrifuged multiple times, there is usually excess material either on the lead or terminal pin, or on the top surface of the capacitor. This requires multiple cleaning steps after the polyimide is cured at an elevated temperature. These cleaning steps typically consist of microblasting using sodium bicarbonate. No matter what the microblasting medium is, multiple cleaning steps are then required. In a typical application, this would mean multiple cleaning and ultrasonic baths containing de-ionized (DI) water followed by alcohol rinses, and subsequently followed by other cleaning solvents. After all of this, the subassembly is subjected to a bake-out process. To make the outside diameter connection to the ferrule, almost all of the above steps are repeated.
All of the foregoing manufacturing steps are highly labor intensive. This was not a significant problem when volumes of implantable medical devices were relatively low. However, in the United States alone, there are over 500,000 pacemakers implanted annually. This market is growing rapidly with the advent of biventricular pacemaking to control congestive heart failure. Thus, high volume manufacturing techniques are needed to control the cost.
Accordingly, there is a need for a manufacturing methodology which advantageously lends itself to high-volume manufacturing techniques. Preferably, such a manufacturing methodology would eliminate many of the foregoing labor-intensive manufacturing steps, and especially those related to the use of a non-conductive polyimide material as an insulating washer. By eliminating the conductive polyimide, one also eliminates the non-conductive bonding washer and all the related centrifuging and cleaning steps. The present invention addresses these needs and provides a very low cost manufacturing methodology for EMI filtered hermetic terminal assemblies for active implantable medical devices.